The present invention relates to a viscous fluid coupling for transmitting rotational torque from an input member to an output member through viscous fluid, and more particularly to a viscous fluid coupling employed in a radiator cooling fan drive mechanism of a vehicle engine.
A conventional viscous fluid coupling is seen in Japanese Patent Laid-Open (KOKAI) publication No. 234931/1991 and includes an input member having a rotatable shaft and a rotatable disc-shaped rotor secured to the shaft; an output member rotatable with respect to an axis of the shaft and having a casing rotatably supported on the shaft and a cover secured at it's outer periphery to the casing to define a hollow interior space, the rotor being housed in the hollow interior space; a partition plate positioned in the hollow interior space between the cover and the rotor for dividing the hollow interior space into a working chamber and a fluid storage chamber, the partition plate secured to the cover and having apertures for allowing the viscous fluid to flow from the storage chamber to the working chamber; a valve plate for closing and opening the apertures formed in the partition plate and secured on a rod which is supported on the cover; a temperature responsive bi-metal spiral spring element secured to the rod for actuating the valve plate to a position to open the apertures formed in the partition plate and a position to close the apertures formed in the partition plate; labyrinth device provided at least between the rotatable rotor and the casing for transferring rotation of the rotor to the casing; and a holder plate secured to the cover and fixedly holding an outer periphery of the spiral spring element.
In this prior case, a welding point to secure the bi-metal spiral spring element to the holder plate is located near a part of the spiral spring element which will be subjected to a concentrated stress. A disadvantage in this type of coupling is the decrease in the strength of the bi-metal spiral spring element due to thermal effect of the welding process.
Specifically, the location of the welding point near the concentrated part of the stress in the bi-metal spiral spring element will amplify such a disadvantage.